Photovoltaic cells were first developed in the 1950's as p-n junctions of inorganic materials. A wide variety of cells since then have been fabricated using homojunction, heterojunction and tandem architectures with inorganic materials, most commonly silicon. As solar cells, the devices convert solar radiation (sunlight) directly into direct-current electrical power. However, widespread terrestrial use of the cells has been impeded by the high peak-watt energy cost compared with that derived from oil, natural gas, and coal.
Amorphous silicon has been contemplated as a highly promising alternative to the more expensive crystalline silicon and efforts have been undertaken at constructing cells from this material. However, at present the best silicon photovoltaic cells are about seven times more expensive than conventional energy sources.
An alternative to silicon-based cells was introduced by Graetzel et al. of EPFL-Lausanne, Switzerland. The cell developed by Graetzel et al. is about as efficient as the best amorphous silicon devices; however, these cells employ a liquid electrolyte which requires that the cells be hermetically sealed. In practice, such sealing can be difficult to achieve. If the cells are not properly sealed, the electrolyte can evaporate with a concomitant decrease in efficiency.
The cell introduced by Graetzel et al. is an example of a relatively efficient photovoltaic device which is fairly simple to fabricate using low-cost materials. The operating principle is based on the dye-sensitization of a wide-band gap metal oxide, nanoporous semiconductor layer. In particular, the layer is formed with an interconnected network of nanocrystals of titanium dioxide coated with a single molecular layer of a light absorbing ruthenium-based dye. When the dye layer absorbs light, electrons are transferred to the nanocrystal conduction band. The charge is transported through a number of nanocrystals in the nanocrystal layer (on the order of microns thick) to a transparent, conducting oxide electrode. The circuit is completed with an electrolyte with a redox couple and a counter electrode impregnated with a platinum catalyst.
Conjugated polymers have been developed which are promising for a variety of electronic device applications, such as FETs, photovoltaic cells, LEDs and lasers. Photovoltaic devices using conjugated polymers blended with C.sub.60 have been formed in both a p-type junction architecture, as well as in an interpenetrating network architecture of semiconductor nanocrystals. However, the efficiencies of these devices tend to be much lower than those of the Graetzel device or silicon devices. Because of such inefficiencies, such devices are not currently viable candidates for widespread commercialization.
The photovoltaic devices in the prior art can be grouped into two basic architectures. The conventional photovoltaic devices (inorganic materials such as silicon) and several of the conjugated polymer devices are planar junction devices. In these devices, the free charge carriers, or excitons, created by light absorption diffuse to the junction interface where they are spatially separated, leading to the photovoltaic effect.
The other group embodies an interpenetrating network architecture and includes the Graetzel cell and the conjugated polymer devices blended with C.sub.60 and semiconductor nanocrystals. In this type of architecture, elements of one type of material, for example semiconductor nanocrystals or C.sub.60 molecules, interpenetrate another material where they are physically and electrically coupled to form a charge-transporting network. The network is necessary in the case of the dye-sensitized nanocrystal device to produce sufficient surface area, and thus dye area, to effect adequate light absorption. In devices formed from conjugated polymers blended with other polymers, nanocrystals or C.sub.60 molecules, the structure and operation of the devices is based on an interpenetrating network wherein elements of the latter materials are electrically interconnected and embedded within the polymer material.
Despite the extensive work previously conducted in the field of photovoltaics, there remains a need for a device which exhibits high efficiency, low cost and ease of fabrication which is formed in a fully solid-state embodiment.